A wide variety of forest products are manufactured from wood fibers. U.S. Pat. No. 5,900,304, which is incorporated herein in it's entirety by reference, focuses upon a class of wood-fiber products that are molded in three dimensions under conditions of heat and pressure to produce a structural wood fiber web that serves as the principal structural component of composite structural-fiberboard panels. The geometry of the web permits the use of a simple rigid mold that may be pressed together with one-dimensional forces. When the fiber web is bonded to sheet coverings or facings to produce a composite panel product, the composite structure forms a strong, lightweight, rigid three-dimensional truss.
Other fiberboard products having three-dimensional elements. For example, U.S. Pat. No. 4,702,870 (“US '870”), which is incorporated herein in it's entirety by reference, describes a method and apparatus for forming three-dimensional structural components from wood fiber. This method and apparatus require the use of a resilient mold insert to form three-dimensional features in the finished fiberboard product. The resilient mold insert is most commonly composed of an array of elastomeric protuberances. The elastomers are attached to a rigid support plate. In mass-production of wood-fiber products, elastomeric mold elements can exhibit problems with compression-set and relatively rapid deterioration under the heat and pressure necessary for product consolidation and drying. As a result, the elastomeric mold elements have a relatively short lifetime and need to be frequently replaced in high-speed production facilities. In addition to short mold lifetimes, the three-dimensional fiberboard objects disclosed are limited to objects having a flat face, backed by webs extending approximately normal to the flat face, backed by webs of identical fiber composition.
Heat transfer from the resilient mold insert of US '870 to the fiber mat is slow because of the low thermal conductivity of the elastomeric elements of the mold inert and because of long thermal-conduction pathways to regions of the fiber between the elastomeric mold elements. Slow heat transfer results in long drying times within the press, a major problem for this method, particularly for thick products. Drying speed may be increased using radiowave heating of the fiber mat, but this increases the complexity and cost of equipment used to form and dry the fiberboard products.
A process for making grids from fibers, described in U.S. Pat. No. 5,277,854, which is incorporated herein in it's entirety by reference, also uses the idea of a resilient mold insert which is capable of forming objects in three-dimensions. Because of the use of a resilient mold insert, this invention suffers from the same difficulties as does US '870. In addition, while this mold insert is capable of generating three-dimensional forces, it is used to generate a fiber product that has generally two-dimensional features only.
U.S. Pat. Nos. 5,198,236 and 5,314,654, which are incorporated herein in it's entirety by reference, describe a method and apparatus that uses a rigid mold to form three-dimensional features in structural fiberboard products. These fiberboard products are limited to flat-faced objects backed by webs extending approximately normal to the flat face. In addition, the rigid mold elements disclosed must be retracted during consolidation of the fiber. In U.S. Pat. No. 5,314,654, a second forming step is required using a resilient mold insert similar to that of US '870. Therefore, formation of this structural fiberboard product suffers from the same difficulties as have been pointed out for US '870. In addition, the need for retractable mold elements makes this method complex and expensive.
U.S. Pat. No. 5,316,828 (“US '828”), which is incorporated herein in it's entirety by reference, reveals a reinforced fluted medium and corrugated fiberboard that has increased strength and stiffness in comparison to conventional corrugated fiberboard due to the addition of three-dimensional elements in a simple corrugated fiberboard structure. The three-dimensional elements take the form of adhesive material applied along lines that are transverse to the flutes. The adhesive at least partially fills in and bridges across the valleys of the flutes, holding the corrugated board more rigid under compressive and bending stresses both along the corrugations and across the corrugations.
The structure of US '828 is therefore not formed as a single piece and requires multiple manufacturing steps. In addition, considerable adhesive is required to fill in the valleys to the top of the flutes. The adhesive could fill in and bridge only a small portion of the flutes in thick corrugated boards, making the technique ineffective for thick corrugated panels. Finally, application of adhesive to both sides of the fluted medium would increase product weight and material cost, and complicate board manufacture.
U.S. Pat. No. 4,726,863, which is incorporated herein in it's entirety by reference, describes a method for making a high-strength composite paperboard panel. The panel is composed of an undulated midstratum layer to which are adhesively bonded an underlayer and an overlayer. There is no variation of the structure along the flutes formed by the undulations, making the structure generally two-dimensional and placing it in a different structural class than the present invention. Because of its two-dimensional structure, which is similar to the structure of conventional corrugated boards, this panel product has less strength and stiffness across the undulations compared to along the undulations.
U.S. Pat. No. 5,900,304, which is incorporated herein in it's entirety by reference, describes a wet-formed three-dimensional core for structural composites. The three-dimensional wood-fiber web can be produced using a simple rigid mold pressed together with one-dimensional pressing forces. The fiber webs are made using a wet-forming process in which the wood fiber is prepared by mixing the fiber with water, thereby forming a slurry. The fiber web is formed as one piece under heat and pressure after most of the carrier fluid is drained or squeezed from the slurry as the rigid mold is pressed together. Once formed using the rigid mold, the fiber web contains corrugations that have syncline (V-shaped) indentations along the ridges of the corrugations on both sides of the web at spaced positions along the ridges of the corrugations. The opposite surface of the indentations form anticline (inverted V shaped) protrusions that function as corrugation stiffeners bridging across furrows of the corrugations. These elements produce sloped web surfaces. The valleys and ridges of these elements may be flat. Flat ridges provide an exterior surface for the application of adhesives that bond the web to additional components. Surfaces that are either sloping or flat allow formation of the web using a simple rigid mold that is pressed together using a one-dimensional pressing force.
In structural panel applications, sheets of material are adhesively bonded to the flat ridges of the shaped web on one or both sides of the web, providing smooth facings which cover the web. The web thereby serves as a stiff, light-weight structural core which is sandwiched between sheet facings to form a composite panel. The combined structure consists of a three-dimensional web bonded to sheet material.
U.S. Pat. No. 6,451,235, which is incorporated herein in it's entirety by reference, describes a wet process for forming a three-dimensional core from an aqueous slurry of fibers employing press drying to produce a strong core structure. Wet-formation techniques can be employed for producing deeply drawn three-dimensional structural cores using what is said to be the natural plasticity and conformability of wet fibers. The use of wet fibers can enable the forming, drawing and bonding of such structures without the use of complicated mold designs and complex formation processes.
A commercial product, SONOBoard™, based on technology described in U.S. Pat. No. 4,702,870, which is incorporated herein in it's entirety by reference, can utilize a complex deformable mold upon which an aqueous slurry of fibers can be formed into a three-dimensional honeycomb with one flat face. The wet pre-form composition can be wet-pressed and then transferred to a hot press wherein it can be press-dried to form a semi-rigid composite structure. Two of these structures can be laminated along the honeycomb face to produce a flat, rigid panel. Another form of this product is commercially known as Gridcore™.